The disclosure relates generally to turbomachine measurements, and more particularly, to sensor systems positioned relative to a circumferential interior surface of a turbomachine casing.
Turbomachines are widely used to generate power. Most turbomachines such as gas turbines, jet engines, steam turbines, etc., are equipped with sensors for the purpose of, for example, monitoring the health of the machine, validating new parts, and/or performing diagnostics. Sensors may be discrete, independent measurement points or they may be discrete measurement points as part of a larger system. The sensors may measure parameters such as temperature, pressure, distance, speed, physical presence of a part, etc. In one particular example, the magnitude and frequency of vibration of a rotating blade may be measured using an array of strategically positioned, stationary, non-contact sensors. This technique is referred to as a “blade tip timing” measurement.
One sensor integration approach requires machining of holes that penetrate radially from the outer diameter of the casing to the inner diameter of the casing. The sensors are mounted in the radial holes. This approach presents a number of challenges. First, the axial and circumferential positions of the sensors (as well as pitch angle relative to radial) is typically critical to the integrity of the measurement. Accordingly, the machining of the radial holes must be performed with such precision that it can typically only be achieved in a controlled setting in a factory or machine shop. Portable tooling for drilling radial holes has been provided, but its use is complex, expensive, and may be unreliable. Furthermore, each radial hole must be oriented to point inward, towards a centerline of rotation of the rotor of the turbomachine. During the machining, the turbomachine half-shell casing is typically separated from the rest of the machine, which requires aiming a machining tool at a virtual point in space, making it very difficult to achieve any level of precision. In this case, the location of the turbomachine centerline must be inferred using other physical features on the half-shell casing. It is also exceptionally difficult, if not impossible, to verify whether the installed probe is truly radially oriented when machining is complete. This uncertainty introduces the possibility of erroneous data or misinterpretation of the measurement.
In many instances, more than one radial hole is required to create an array of sensors to attain more information, e.g., six to twenty per stage. Consequently, portable tooling requires a new setup for each and every radial hole, including checks prior to performing the machining. This process is incredibly time consuming, and prevents quick turnaround to return the turbomachine to operation. However, where a number of sensors are employed, the number of sensors has to be limited to prevent diminishing the mechanical integrity of the casing. Furthermore, irregular or asymmetric holes patterns are typically avoided because they can create non-uniform stress distributions.
Another challenge with conventional sensor positioning includes avoiding drilling into the many possible obstacles on the exterior of the casing. Obstacles may include pipes, insulation, flanges, lifting lugs, other instrumentation, bolts, or any other physical object in close proximity to the casing. These obstacles may prevent the positioning of a sensor in the optimal location, possibly jeopardizing the measurement. In addition, the tooling can be quite heavy and difficult to move. It is also common practice to remove unnecessary sensors from a turbomachine when they are not needed to reduce possible leak locations. To reduce the risk of a leak, it is typical for the sensors to be removed and the opening plugged with a more robust device.
Another challenge with the current sensor approach is that it prevents the use of two measurement points or two different types of sensors in the same location because it is typically not feasible to drill two or more radial penetrations in the casings within a prescribed distance from one another. When sensors are oriented radially, projecting outward from the outer surface of the casing, the often delicate instrumentation is highly susceptible to damage.
Another sensor integration approach provides passive sensors on the rotating blade inside the casing. Typically, such sensors are powered by circumferentially spaced power transmission elements, e.g., coils, and antennae. These sensors provide multiple, intermittent measurements as the rotating blade rotates, i.e., once per revolution. Obtaining useful data on quickly changing physical properties such as strain, requires measurements to be completed at a very high frequency, e.g., 300 MHz, which cannot be achieved on a per revolution basis. Current passive sensors also must be very close to the antenna that receive data from the sensors in order for them to work property, which can be very challenging on a turbomachine.